The present invention relates generally to discharge lamp lighting devices which can have a high start performance and more particularly, to a discharge lamp lighting device which can improve a start performance of a high-intensity discharge lamp (HID lamp) and reduce stresses in constituent parts of the lamp.
A prior art discharge lamp lighting device disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 63-150895, comprises first and second transistors for high frequency operation, a third and fourth transistors for low frequency operation, a control circuit for controlling ON/OFF operation of the first to fourth transistors, and an ignitor for generating a high pulse voltage, a rectangular-wave shaped A.C. voltage having polarities inverted at intervals of a constant time being applied to a discharge lamp.
The control circuit in the aforementioned device includes first and second comparators to which reference voltages Vr1 and Vr2 are applied, an oscillation circuit for oscillating a clock signal having a predetermined frequency set by a time-constant circuit of a capacitor and a resistance, a first flip-flop inverted by outputs of the comparators, a timer circuit for outputting a pulse signal having a constant period, and a drive circuit for generating a control signal for two pairs of transistors on the basis of an output (high frequency signal fh) of the flip-flop and an output (low frequency signal fl) of the timer circuit.
The drive circuit includes a second flip-flop, first to fourth AND circuits, fifth and sixth transistors, and first and second pulse transformers.
The ignitor, which includes a bi-directional 3-terminal thyristor (triac), a bi-directional 2-terminal switch (switch) and a third pulse transformer, is arranged so that, at the time of starting the discharge lamp, turning ON and OFF of the triac causes a starting high pulse voltage induced in a secondary winding of the third pulse transformer to be applied to the discharge lamp.
Explanation will be made as to the operation of the above discharge lamp lighting device. An output of the oscillation circuit is compared by the second comparator with the second reference voltage Vr2. When the oscillation circuit output is larger than the second reference voltage, the second comparator produces a high level output, the first flip-flop is set and produces a high level output. From this time moment, any one of the first and third transistors is turned ON so that a current flows therethrough and a voltage Vdt corresponding to the voltage appears between both ends of a current detection resistance. The voltage Vdt is compared by the first comparator with the reference voltage Vr1. When Vdt greater than Vr1, the output of the first comparator is changed to its high level to reset the first flip-flop.
The high frequency signal fh issued from the first flip-flop is applied to the drive circuit as a timing signal for the high frequency switching operation. The low frequency signal fl issued from the timer circuit, on the other hand, is applied to the drive circuit as a timing signal for the low frequency switching operation and is set to have a frequency corresponding to twice its polarity inverting frequency.
The drive circuit frequency-divides the low frequency signal fl by the second flip-flop and third and fourth AND circuits to form an ON control signal for the third and fourth transistors and outputs the signal. The drive circuit also sets the outputs of the first and second AND circuits suitably at high levels on the basis of the high frequency signal fh, and outputs an ON/OFF control signal for the first and second transistors through the fifth and sixth transistors and second and third pulse transformers. Accordingly an A.C. voltage, which is supplied from an inverter of a full bridge type and applied to the discharge lamp via the inductor, is a voltage a polarity of which is inverted at intervals of a constant time and which is chopped at a high frequency.
The ignitor is arranged so that a second capacitor is already charged before its polarity inversion, a third capacitor is already charged through the resistance after the polarity inversion, and the triac is made conductive depending on a time constant determined by the first and second resistances and a fourth capacitor. That is, when a voltage across the fourth capacitor reaches a switch response voltage, the switch is made conductive to trigger the triac. When the triac is made conductive in this manner, the second and third capacitors are connected in series so that electric charges so far accumulated in the capacitors are discharged through the primary winding of the first transformer and a starting high pulse voltage is generated in the secondary winding thereof to be applied to the discharge lamp.
Such a prior art discharge lamp lighting device as mentioned above, however, has had a problem that the polarity inversion period of the A.C. voltage applied to the discharge lamp in the lamp start mode (discharge start mode) is equal to that in the stationary lighted mode (switching frequency of 100 to 200 Hz), so that, even when the high pulse voltage is applied to start the discharge lamp in the start mode, its inverted polarity voltage is immediately applied to the discharge lamp, which undesirably results in that it is hard for the lamp to maintain the discharge, the lamp cannot be smoothly shifted to its stationary lit state, with a bad startability.
A control circuit, which is proposed as one of measures of improving the aforementioned known device and which includes, in addition to the aforementioned arrangement, a third flip-flop and elements connected thereto, is arranged so that, in a no-load mode wherein the discharge lamp is not started yet, the output of the first comparator is set always at a low level to avoid accumulation of electric charges in its ambient capacitors, and when the transistor is turned OFF and the output of the third flip-flop is changed to its low level, a third resistance is inserted to cause the A.C. voltage to have a sufficiently long polarity inversion period (of, e.g., 10 Hz or less). Therefore, in the case of this discharge lamp lighting device, when the high pulse voltage is applied to almost start the discharge, an identical polarity of voltage is applied for a sufficient long time, whereby the discharge lamp can be easily shifted to its stable discharge state and its startability can be improved. However, the discharge lamp lighting device having the control circuit as such an improving means has still had a problem that the polarity inversion period of the A.C. voltage is long with respect to the entire lamp start time, with the result that an interval between pulses in the high pulse voltage applied to the discharge lamp becomes long and thus its starting time becomes long.
According to another solving means, an LC resonance voltage is used to generate the high pulse voltage. For example, when a set of the first and fourth transistors and a set of the second and third transistors are alternately turned ON and OFF with an On duty of 50%, a high pulse voltage having an equal and continuous level by LC resonance can be applied to the discharge lamp. However, this system has a problem that the voltage applied to the discharge lamp contains no D.C. component, which results in that there cannot be obtained an energy necessary for smoothly shifting the lamp to its arc discharge state after dielectric breakdown.
Further, in order to obtain a high pulse voltage, it is required to set the switching frequency at a value close to the LC resonance frequency. In this case, however, it is necessary to feed a large resonance current, which involves a problem that coils, capacitors, switching elements, etc. undergo great stress.
In view of the above problems in the prior art, the present invention has been proposed. It is a major object of the present invention to provide a discharge lamp lighting device which can suitably start and light a discharge lamp with a high pulse voltage obtained through LC resonance, can supply to the discharge lamp an energy necessary for smoothly shifting the lamp to an arc discharge state to improve its startability, and also can reduce costs of constituent parts such as coils, capacitors and switching elements.
The object of the present invention is attained by providing a discharge lamp lighting device which comprises a D.C. power source circuit having a pair of output terminals for supplying a D.C. power; a polarity inverting circuit including a series circuit of at least first and second switching elements connected in parallel with the output terminals of the D.C. power source circuit for converting a D.C. power received from the D.C. power source circuit into an A.C. power; a load resonance circuit including an inductor, a capacitor and a discharge lamp connected in parallel with the capacitor, and a control circuit for alternately turning ON and OFF the first and second switching elements of the polarity inverting circuit to control a voltage to be applied to the discharge lamp of the load resonance circuit, wherein the control circuit alternately turns ON and OFF the first and second switching elements at a high ON/OFF frequency so as to alternately provide a first period during which an ON duration of the second switching element is longer than that of the first switching element and a second period during which the ON duration of the second switching element is shorter than that of the first switching element, to thereby apply a low frequency voltage of a rectangular waveform to the discharge lamp, superimposes a D.C. component upon resonance pulse signal received from the load resonance circuit in the ignition mode of the discharge lamp, and continuously changes a switching frequency of the switching elements to apply a high voltage to the discharge lamp.